Four people have been arrested after the discovery of a large fuel laundering operation in Co Monaghan and raids on filling stations in five other counties.

Thousands of litres of laundered fuel were seized at filling stations in in Roscommon, Galway, Offaly, Westmeath and Dublin.

Gardaí also uncovered a "significant oil laundering plant" was near Castleblaney in Co Monaghan.

The plant had the capacity to launder an estimated 18m litres of fuel per year. which Revenue said represented a potential loss to the Exchequer of €9m a year.

A laundering operation was under way at the time of the raid and around 95,000 litres laundered fuel were seized at the site.

Three tankers, two transit vans, a box lorry, a 40 foot tanker and an articulated truck were also recovered.

A man in his 30s from Northern Ireland was arrested at the scene.

The PSNI later carried out four searches in Armagh and Tyrone.